1. Field of the Invention
The present invention relates to a standard mechanical interface (SMIF) load port assembly including a so-called xe2x80x9csmartxe2x80x9d port door, and in particular to a SMIF load port assembly including a port door position compensation assembly capable of dynamically adjusting a relative spacing between a front surface of a port door and a front surface of a pod loaded onto the load port assembly so as to compensate for any misalignment of the front surface of the pod.
2. Description of Related Art
A SMEF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
The SMIF system provides a clean environment for articles by using a small volume of particle-free gas which is controlled with respect to motion, gas flow direction and external contaminants. Further details of one proposed system are described in the paper entitled xe2x80x9cSMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSIMANUFACTURING,xe2x80x9d by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (xcexcm) to above 200 xcexcm. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half xcexcm and under. Unwanted contamination particles which have geometries measuring greater than 0.1 xcexcm substantially interfere with 0.5 xcexcm geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 xcexcm and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafer cassettes; (2) a minienvironment supplied with ultraclean air flows surrounding cassette load ports and wafer processing areas of processing stations so that the environments inside the pods and minienvironment become miniature clean spaces; and (3) robotic transfer assemblies, such as load ports, to load/unload wafer cassettes and/or wafers from the sealed pods to the processing equipment without contamination of the wafers in the wafer cassette from external environments. The system provides a continuous, ultraclean environment for the wafers as they move through the wafer fab.
SMIF pods are in general comprised of a pod door which mates with a pod shell to provide a sealed environment in which wafers may be stored and transferred. So called xe2x80x9cbottom openingxe2x80x9d pods are known, where the pod door is horizontally provided at the bottom of the pod, and the wafers are supported in a cassette which is in turn supported on the pod door. It is also known to provide front opening unified pods, or FOUPs, in which the pod door is vertically oriented, and the wafers are supported either in a cassette mounted within the pod shell, or to shelves mounted in the pod shell. Front opening pods include a door having a rear surface which is included as part of the sealed pod environment, and a front surface which is exposed to the environment of the wafer fab.
FIG. 1 is a prior art perspective view of a 300 mm front opening SMIF pod 20 including a pod door 22 mating with a pod shell 24 to define a sealed environment for one or more workpieces located therein. FIG. 2 is a prior art perspective view of a 300 mm load port assembly 23 for transferring wafers between the pod 20 and a process tool 28 to which the load port assembly 23 is affixed. In order to transfer the workpieces between pod 20 and process tool 28, the pod is manually or automatedly loaded onto a pod advance plate 25 so that a front surface 31 of the pod door faces a front surface 30 of a port door 26 in the load port assembly.
The front surface 30 of the port door 26 includes a pair of latch keys 32 which are received in a corresponding pair of slots 33 of a door latching assembly mounted within pod door 22. An example of a door latch assembly within a pod door adapted to receive and operate with latch keys 32 is disclosed in U.S. Pat. No. 4,995,430 entitled xe2x80x9cSealable Transportable Container Having Improved Latch Mechanismxe2x80x9d, to Bonora et al., which patent is assigned to the owner of the present invention, and which patent is incorporated by reference herein in its entirety. In order to latch the pod door to the port door, the pod door 22 is seated adjacent the port door 26 so that the vertically oriented latch keys are received within the vertically oriented slots 33.
In addition to decoupling the pod door from the pod shell, rotation of the latch keys 32 also lock the keys into their respective slots 33, thus coupling the pod door to the port door. There are typically two latch key 32 and slot 33 pairs, each of which pairs are structurally and operationally identical to each other.
The pod advance plate 25 typically includes three kinematic pins 27, or some other registration feature, which mate within corresponding slots on the bottom surface of the pod to define a fixed and repeatable position of the bottom surface of the pod on the advance plate and load port assembly.
Referring to FIG. 3, the pod advance plate 25 is translationally mounted to advance the pod toward and away from the load port. Once a pod is detected on the pod advance plate by sensors in the load port assembly, the pod is advanced toward the load port in the direction of arrow Axe2x80x94A until the front surface 31 of the pod door 22 lies in contact with the front surface 30 of the port door 26. It is desirable to bring the front surfaces of the respective doors into contact with each other to trap particulates and to ensure a tight fit of the port door latch key in the pod door key slot. However, some process tool manufacturers require that a small space be provided between the port plate surrounding the port door and the pod shell flange at the front edge of the pod shell after the pod has advanced. This space prevents any possible contact between the port plate and the front surface of the pod due to a misaligned front pod surface as explained below.
Once the pod and port doors are coupled, horizontal and vertical linear drives within the load port assembly move the pod and port doors together toward the process tool, and then away from the load port so that wafers may thereafter be transferred between the interior of the pod 20 and interior of process tool 28.
Regardless of the desired relative positions of the pod and port doors after pod advance, it is necessary to precisely and repeatably control this relative positioning to ensure proper transfer of the pod door onto the port door and to prevent particulate generation. hI order to establish the desired relative positions, conventional load port assembly systems rely on the fact that the kinematic pins establish a fixed and known position of the pod on the load port assembly so that, once seated on the kinematic pins, the pod may simply be advanced toward the load port a fixed amount to place the front surfaces of the respective doors in the desired relative positions.
However, it is a drawback to conventional front opening load port assemblies that the system aligns the bottom surface of the pod to the load port assembly by the kinematic pins, but registers off of the front surface of the pod in establishing the proper positioning of the pod door to the port door. The problem is that the actual position of the front surface of a pod seated on a load port assembly may vary as much as approximately 1 mm in front of or behind the expected position. The sources of this variance include warping and tolerances of the pod and/or pod door, the tolerances in the location of the kinematic pins and the tolerances in the location of the port door in the load port.
If the front surface of the pod door is farther from the port door than expected, then the port door latch key may not properly seat in the pod door key slot upon pod advance. This can result in damage to the pod door cover by the latch keys, can create particulates and can cause pod door opening failure resulting in production stoppage and delays. If the front surface of the pod door is closer to the port door than expected, then contact between portions of the pod and the load port upon pod advance may generate particulates.
It is therefore an advantage of the present invention to allow contact or close proximity of the front surfaces of the pod and port doors.
It is another advantage of the present invention to provide precision control of the spacing between the port and pod doors.
It is a further advantage of the present invention to compensate for any variation between the expected and actual positions of the pod door prior to coupling of the pod door to the port door.
It is a further advantage of the present invention to reduce the risk of damage to the pod door resulting from incorrect seating of the port door keys in the pod door slots.
It is a still further advantage of the present invention to reduce the risk of particulate generation resulting from unintended contact between portions of the pod and the load port assembly upon pod advance to the port.
It is another advantage of the present invention to minimize production downtime resulting from incorrect seating of port door keys in pod door slots.
It is a further advantage of the present invention to allow the pod door to be returned to its correct position with respect to the pod shell after wafer processing within the processing tool.
These and other advantages are provided by the present invention which in general relates to a SMIF load port assembly including a port door position compensation assembly capable of dynamically adjusting a relative spacing between a front surface of a port door and a front surface of a pod door loaded onto the load port assembly so as to compensate for any improper positioning of the front surface of the pod.
In one embodiment of the invention, the door position compensation assembly includes a plunger having a back end mounted within the port door and a front end protruding past a front surface of the port door. The plunger is translationally mounted so as to be able to retract into the port door upon a force exerted on the front end of the plunger. The position compensation assembly further includes a sensor capable of sensing movement of the plunger. In one embodiment, this sensor may comprise a resistor sensing circuit including a potentiometer having a resistance variation actuator to which the back end of the plunger is affixed. After a pod has been loaded onto the pod advance plate of the load port assembly, and is advancing toward the port door, a front surface of the pod door will contact the front end of the plunger to move the plunger rearward at least partially into the port door. Rearward movement of the plunger actuates the resistance variation actuator to thereby change the resistance of the potentiometer, and consequently the voltage across the variable resistance sensing circuit.
The overall system controller uses the electrical change (either voltage, resistance or current) in the sensing circuit of the position compensation assembly to provide a closed loop servo control and positioning of the port door with respect to the pod door while the pod door is advancing toward the port door, or after the pod door has reached its fully advanced position. The controller for the system stores the relationship between the electrical change through the sensing circuit and position change of the plunger. The controller also stores the value of, for example, the voltage across the circuit when the front surfaces of the pod and port doors are in contact with each other. Using this stored relationship and stored value, the position compensation assembly and the controller employ a closed loop servo positioning and control system to determine the final resting positions of the port and/or pod doors.
In one embodiment, after the pod advance plate has moved the pod to its fully advanced position, the controller samples the voltage across the resistor sensing circuit, determines the difference between the sampled voltage and the desired final voltage, and adjusts the position of the port door through closed loop servo control until the final desired voltage is obtained. The controller and position compensation assembly may further determine and adjust the final positions of the port and/or pod doors to obtain contact between the pod and port doors according to other control algorithms.
In a further embodiment of the present invention, sensors as described above may also be provided in the port plate surrounding the port door to sense the position of the pod shell. It is desirable that the pod shell be close to but not in contact with the port plate. The sensors in the port plate can detect the position of the pod shell relative to the port plate, and then use this information to adjust the position of the pod. To the extent this adjustment changes the position of the pod door relative to the port door, the position of the port door may be adjusted to ensure proper positioning therebetween.